Not to mention, the CSA would need a Casus Belli. What would it possibly be? They have land, we want it? I guess they can just simply declare war...good luck being trusted by other countries because it would now be super easy for any nation to justify material aid to Mexico....like say the USA.
Well, that's why I came down pretty hard on the side of "Why would they do that?"
I'm just not sure there'd be enough interest in trying to push west past Texas... of course, the scenario I discussed upthread speculated on a much more successful battle of Glorietta Pass for the South, and one in which the CSA controlled everything east and south of the Colorado River by the time of an armistice. I guess I could see some real justification for "the gulf of California is within spitting distance, we just need a few more miles of territory and we'll have that pacific port," in that scenario.
As far as casus belli goes, many countries of the 19th century were rather spotty on it. Anything from a "Columbus,NM" style raid, to "protecting our citizens down there," to "defaulted loans," to even a false flag are all justifications that worked in that day and age. But you're point is well taken.